1. Field of the Invention
The present invention relates to a mass spectrometry system and mass spectrometry method using a mass spectrometer.
2. Description of the Related Art
In recent years, a general method for identifying a protein with a mass spectrometry method has been carried out by using a tandem mass spectrometer. In this technique, a measuring target sample is separated by a liquid chromatograph, and thereafter ionized. The thus-generated ions are introduced into a mass spectrometer, and separated therein according to mass-to-charge ratios m/z (hereinafter simply described as “m/z”), so that intensities of the ions are detected. Such an analysis method is called as first mass spectrometry. Obtained data is processed by a computer and the processed data is outputted as data such as an MS spectrum. In the tandem mass spectrometer, one of ions having a specific m/z value is selected as a precursor ion. Here, the m/z value is obtained through the first mass spectrometry method. Ions are dissociated from the selected precursor ion with a method termed as collision-induced dissociation (CID) or other methods. In the CID, the precursor ion is firstly made to collide with molecules of an inactive gas, and is activated by partially converting the collision energy into internal energy. Then, consequently, the ions are dissociated from the precursor ion. A method for detecting product ions produced by the dissociation of the precursor ion is called as second mass spectrometry. A protein is identified by comparing an MS2 spectrum obtained through the second mass spectrometry method with a theoretical spectrum obtained from sequence information of known proteins by using a statistical method.
As a protein quantification method using a mass spectrometry, an internal standardization method is often used instead of an absolute quantification method. In the internal standardization method, internal standard substances labeled with stable isotopes in advance are added to a sample. For example, two samples including a specific amino-acid residue (cysteine) are chemically labeled using Cleavable ICAT (registered trademark) Reagents made by Applied Biosystems and a relative quantitative analysis is performed on the two samples. This method makes it possible to minimize variations in a recovery rate during pretreatment and variations in ionization during mass spectrometry and to perform relative quantitative analysis with high accuracy. A relative quantification ratio is calculated by using a peak area in an MS spectrum of labeled peptide ions. In this case, it is not always necessary to identify both ions of an isotopically labeled pair.
Analysis techniques using the aforementioned protein identification method and relative quantification method are often used to comprehensive study, with an approach called proteomics, on proteins present in blood (blood plasma and serum), urine, organ and the like with an approach called proteomics. Under such circumstances, there has been explored a method for performing comprehensive analysis on samples with high-throughput. However, high-throughput proteomic approach has not been fully achieved yet because of various difficulties. One of the reasons for such difficulties is that when mass spectrometric measurement is performed to identify and quantify proteins in a short time, sufficient time required for the second mass spectrometry method is not ensured since too many constituents are ionized simultaneously. Accordingly, some ions remain unanalyzed after the measurement. Moreover, when the relative quantitative analysis is performed on two samples labeled with the aforementioned stable isotope elements, the number of ions produced by the ionization is doubled. This makes it more difficult to ensure time necessary to perform the second mass spectrometry on the ions. For this reason, in the present circumstances, some contrivance such as separation of the sample is made in the stage of sample preparation to reduce the number of ions to be ionized simultaneously. However, this approach has a disadvantage of increasing measurement time.
In addition to the contrivance in sample preparation and the development of device, a control method is being developed in which only an analysis-target precursor ion is efficiently subjected to the second mass spectrometry among multiple ions simultaneously produced by the ionization. Specifically, the development of this control method is attempted by improving the algorithm of control software for selecting the target ion to be subjected to the second mass spectrometry.
Many reports relevant to the aforementioned control software have been published.
According to International Patent Publication WO 2002/025265, a determination is made as to whether or not an ion having a specific m/z value is present in an MS spectrum obtained through first mass spectrometry, and then second mass spectrometry is performed in accordance with the determination result. According to Japanese Patent Application Publication No. 2006-329881, intensity information included in an MS spectrum obtained through first mass spectrometry is effectively used to optimize an analysis flow including selection of a precursor ion to be subjected to the next second mass spectrometry. According to Japanese Patent Application Publication No. 2005-345332, an MS spectrum obtained by performing first mass spectrometry on samples labeled with different stable isotope elements, is analyzed in real time to determine a precursor ion to be subjected to the next second mass spectrometry. According to Japanese Patent No. 3766391, a determination is made as to whether or not to perform a third-order mass spectrometry, which is a third-stage mass spectrometry, on the basis of m/z peak information of an MS spectrum obtained through second mass spectrometry. According to Japanese Patent Application Publication No. 2006-053004, when the same sample is repeatedly measured by using a liquid chromatograph under the same separation condition, precursor ions are selected and measured as follows. At a first round of measurement, a precursor ion is automatically selected by using, as an index, intensity of an ion peak in an MS spectrum obtained through first mass spectrometry. At the same time, mass information on the precursor ions subjected to the second mass spectrometry and retention time before the elusion of the precursor ions from the liquid chromatograph are automatically registered in an internal database. At second and following rounds of measurement, the same precursor ions are not subjected to the second mass spectrometry and an ion having the next highest intensity is measured.